I searched and could not find this error posted.
May not be a big deal, but just posting in case it can be fixed.
I can understand if this isn't important enough to fix in 5 and have to wait for 6 to be done.

Not sure, but this looks like the same blizzard bug with tainted functions we've seen in other cases. In this particular case, it also looks to me like you may have the bagbar set to hide except when you mouse over it (can you do that? I don't remember). If so, you probably moved the mouse over the bagbar's location while you were (or while the client thought you were) in combat. I suspect you'll have to wait for nUI6 for the fix.